Fabrication materials and techniques for plate heat and mass exchangers for indirect evaporative coolers

ABSTRACT

Heat exchanger plates for indirect evaporative coolers, of the type having a dry side having low permeability to an evaporative liquid and formed to allow a product fluid to flow over a heat transfer area of its surface, a wet side designed to have its surface wet by an evaporative liquid, and formed to allow a working gas to flow over its surface to evaporate the evaporative liquid, are formed such that the wet side comprises a hydrophobic fiber sheet and the dry side comprises a non-permeable sealing layer on the sheet. Heat seal strips are formed at the inlet and outlet of the plates and air flow perforations are formed through the plates.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 6,581,402, issued Jun. 24, 2003 is incorporated herein by reference. U.S. Pat. No. 6,705,096, issued Mar. 16, 2004 is incorporated herein by reference. U.S. Pat. No. 7,228,699, issued Jun. 12, 2007 is incorporated herein by reference. This application claims the benefit of U.S. Provisional Patent Application No. 61/135,640, filed Jul. 21, 2008.

1. Field of the Invention

The present invention relates to plate heat and mass exchangers for indirect evaporative coolers. In particular, the present invention relates to improved fabrication materials and techniques for such plate heat and mass exchangers for indirect evaporative coolers.

2. Discussion of the Background Art

Indirect evaporative cooling is a method of cooling a fluid stream; usually air, by evaporating a cooling liquid, usually water, into a second air stream while transferring heat from the first air stream to the second. The method has certain inherent advantages compared to conventional air conditioning: low electricity requirements, relatively high reliability, and the ability to do away with the need for refrigerants such as R-134 and all the disadvantages they entail.

U.S. Pat. No. 6,581,402 shows a number of embodiments for indirect evaporative cooling using plate apparatus. FIG. 1 (Prior art) shows a perspective and schematic representation of two plates showing the wet side channels formed by the wet sides of a first and a second plate opposing each other, with their passages oriented in the same general area and illustrating the working gas entering on the dry side, passing through the passages and into the wet side channels. The product fluid is separated from the working gas as they pass along the dry side of the first and second plates. Additional plates form a stack, and adjacent plates have their dry sides facing each other. Thus, the stack of plates would have every odd plate oriented with its dry side facing the same direction and opposite of all even plates.

The invention of U.S. Pat. No. 6,581,402 provides an indirect evaporative cooler having cross flowing wet and dry channels on opposite sides of a plurality of heat exchange plates which allow heat transfer through the plates. The plates include edge extensions to facilitate the removal of water (or similar evaporative fluid) and dissolved minerals from the plates.

For purposes of both U.S. Pat. No. 6,581,402 and the present application, we wish to define certain terms. Refer to FIG. 1, (Prior Art).

1. The wet side or wet portion of the heat exchange surface means that portion having evaporative liquid 22 on or in its surface, thus enabling evaporative cooling of the surface and the absorption of latent heat from the surface. 3. The dry side or dry portion of the heat exchanger means that portion of the heat exchanger surface where there is little or no evaporation into the adjacent gas or fluid. Thus, there is no transfer of vapor and latent heat into adjacent gases. In fact, the surface may be wet but not with evaporative fluid or wet by condensation, and no substantial evaporation occurs. 4. The working stream or working gas 2 is the gas flow that flows along the heat exchange surface on the dry side, passes through the passages 11 in the surface to the wet side and picks up vapor and by evaporation, taking latent heat from the heat exchange surface and transporting it out into the exhaust. In some embodiments, the working stream may be disposed of as waste and in others it may be used for special purposes, such as adding humidity or scavenging heat. 5. The product stream or product fluid stream 1 is the fluid (gas, liquid or mixture) flow that passes along the heat exchange surface on the dry side and is cooled by the absorption of heat by the working gas stream on the wet side absorbing latent heat by the evaporation in the wet area.

The plate also has passageways or perforations 11 or similar transfer means between the dry side of the plate and the wet side in defined areas providing flow from the dry working channels to the working wet channels in which direct evaporative cooling takes place.

The method makes use of the separation of a working gas flow 2 (that is used to evaporate liquid 22 in the wet channels and thus to cool the wet surface of the heat exchanger plate) from the product fluid flow 1, flowing through dry product channels 3 and dry working channels 4 respectively on the same side of the heat exchange plate. Both give up heat to the heat exchange plate that on its obverse surface is being cooled by evaporation in the working wet channels 5.

The working gas flow first enters the dry working channel 4 and then through perforations 11, pores or other suitable means of transfer across the barrier of the plate to the wet side and thence into the wet working channels 5 where evaporation of liquid on the wet channel surface cools the plate.

The dry product channels 3 are on the dry side of this plate. The plate is of a thin material to allow easy heat transfer across the plate and thus to readily allow heat to transfer from the dry product channel to the wet working channel. This is one basic unit or element of the invention illustrating the method of the separation of working gas flows to indirectly cool the separate product fluid by evaporative cooling.

Many evaporative cooling embodiments include a wicking material 25 for distributing the water or other evaporative liquid over the plate wet side. See, for example, FIG. 7 of U.S. Pat. No. 6,581,402, wherein a wicking material 7 distributes the evaporative liquid along wet side channels 5. Plates 6 form a “V-shape” in the embodiment of FIG. 7. Water also evaporates better from a wicking surface than from a water surface, as the wick material breaks down the surface tension of the water.

Wicking up a vertical surface will insure no excess water on the plate surface but also limits the height of the plate that can be used. Wicking water down a surface aided by gravity may be good from a wetting perspective if the amount of water does not exceed what the wick can transport. Wicking in a more horizontal direction can allow a vertical reservoir wetting system such as shown in U.S. Pat. No. 6,705,096. There are some plate heat and mass exchanger applications that require a more innovative geometry that corresponds to a more complicated thermodynamic design that again require a more horizontal application such as U.S. Pat. No. 6,581,402. In most cases creating a means to insure that the wick will not be over run by water is desired.

U.S. Pat. No. 7,228,699 teaches apparatus and methods for drawing excess liquid and minerals away from the heat exchanging portion of the plate, and removing them from the plate. Edge extensions are added to the plates of indirect evaporative coolers to allow excess evaporative liquid to migrate to the edges of the plates and drip off, taking dissolved minerals with it. Better evaporation and heat transfer are also accomplished.

The indirect evaporative coolers of U.S. Pat. Nos. 6,581,402 and 7,228,699 work well. But improvements to fabrication techniques and materials can make the devices work better in some environments. For example, improved materials properly fabricated can resist mold and provide better wicking and evaporation. Therefore, a need remains in the art for to improved fabrication materials and techniques for plate heat and mass exchangers for indirect evaporative coolers.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide improved fabrication materials and techniques for such plate heat and mass exchangers for indirect evaporative coolers. An improved process for fabricating a Dew Point Evaporative Air Conditioner uses the thermo forming properties of a plastic polymer to create a seal edge on the inlet and outlet faces of a heat and mass exchanger plate and optionally around the air passageways between the dry side of the plate and a wet side. The plastic polymer material allows the edges to be sealed by melting the fiber material together eliminating the need for a second sealing material such as epoxy, as is used in previous manufacturing processes.

In addition, the improved material uses a hydrophobic plastic polymer such as polypropylene or a combination of plastic polymer hydrophobic material bonded to the top of a hydrophilic material such as a polypropylene spun bond intermingled or on top of a nylon naturally wicking material, with the hydrophobic material on the evaporative surface to improve wicking and evaporation, and retard mold.

In a preferred embodiment, a surfactant is used to start the wicking on the hydrophobic layer. The sealing process may be accomplished using an automated die stamper that heats the sealed area to the effective temperature and also cuts the sheets to the desired configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) is a perspective and schematic representation of a plate heat and mass exchanger for indirect evaporative cooling.

FIGS. 2A-D illustrate an embodiment of the invention. FIG. 2A is a plan view of a plate according to the present invention. FIG. 2B is a plan view of a “wet” plate with wet side channels. FIG. 2C is a plan view of a “dry” plate with dry side channels. FIG. 2D is a side view of the plates of 2B and 2C.

FIG. 3 is a flow diagram illustrating a possible fabrication process for a heat exchange plate according to the present invention.

FIG. 4 is a perspective and schematic representation of another embodiment of the present invention, having slanted edge extensions.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 2 and 4A-D show embodiments of improved fabrication materials and techniques for heat transfer plates in indirect evaporative coolers. FIG. 3 shows an example of a fabrication process according to the present invention. While several embodiments are shown and discussed, it will be apparent to those skilled in the art that many other indirect evaporative cooler plate configurations are possible. U.S. Pat. Nos. 6,581,402, 6,705,096, and 7,228,699, incorporated herein by reference, show a variety of plate configurations, and others are known as well.

The following table lists reference numbers used in this patent:

 1 dry side product fluid (e.g. air)  2 working gas (e.g. air)  3 dry side product channels  4 dry side working channels  5 wet side channels  6A wet plates  6B dry plates  7A wet channel guides  7B dry channel guides  9 dry sides of plates 10 wet sides of plates 11 perforations 18 seals 22 evaporative fluid (e.g. water) 23 trough for wetting plates 24 non-permeable side 25 wicking side 26 trough

FIGS. 2A-D illustrate an embodiment of the invention. FIG. 2A is a plan view of a plate 6 according to the present invention. Plate 6 of FIG. 2A does not have channel guides applied, and can be used as a wet plate or a dry plate. It does have perforations 11 and end seals 18 which are formed at the inlet and outlet ends of plate 6 (perpendicular to the lines of perforations 11). Note that perforations 11 are not required in wet plates 6A if they are provided in dry plates 6B (or vice versa) but may be used in both. Note also that the term “wet plate” is used to indicate the plates having channels guides 7A on the wet side of the plate, and the term “dry plate” is used to indicate the plates having channels guides 7B on the dry side of the plate. Naturally each plate has a wet side and a dry side in use. Refer to FIG. 3 and the discussion of FIG. 3 for a description of how the plates might be fabricated.

In one preferred embodiment, plate 6 is formed of a sheet of hydrophobic polypropylene spun bond material forming a fiber surface, with a hydrophilic polypropylene/polyethylene extruded seal layer on the other side. The sheet is about 20″×19.5″, and 0.01″ thick. The spun bond acts as a wicking material 25. The evaporative rate off of a hydrophobic woven or spun bond material is higher than for a hydrophilic material, resulting in about the same temperatures but with ⅓ higher air flow rates than previous indirect evaporative coolers. The polypropylene/polyethylene extruded seal layer forms the non-permeable side 24 of the sheet. Seals 18 are formed by heat staking, or applying heat to melt the fibers.

FIG. 2B is a plan view of a wet plate 6A with wet side channels 5. Seals 18 were formed at the inlet and outlet ends of plate 6A. Channel guides 7A run perpendicular to the seals 18, and the channel guide at each end is formed on top of the seal at that end. In this embodiment, the wet channel spacing (between plates) is about 0.09″, and the distance between the channel guides is about 1″. Since the hydrophobic wicking material 25 will not hold or absorb water naturally, the wet channel spacing can be less than half that of previous indirect evaporative coolers without the wet channels filling up with water and preventing air flow. Wet channel spacing can also be larger if desired for air flow.

FIG. 2C is a plan view of a dry plate 6B with dry side (non-permeable side 24) product channels 2 and dry side working channels 3. Dry channels 2, 3 run parallel to the lines of perforations 11, and perpendicular to end seals 18. In this embodiment, dry channel spacing is about 0.14″ and the distance between channel guides 7B is about 1″.

FIG. 2D is a side view of a plate 6A of FIG. 2B and a plate 6B of FIG. 2C, shown in a stacked configuration as they would be used in an indirect evaporative cooler, but exploded to show detail. Wet plate 7A includes perforations 11 aligned with working channels 4. Plates 6A and 6B have been formed into V-shaped troughs to provide evaporative fluid 22 to wet the wet sides of the plates.

FIG. 3 is a flow diagram illustrating a possible fabrication process for a heat exchange plate according to the present invention. In step 302, sheets of the material for plate 6 are formed. As discussed with respect to FIG. 2A, plate 6 may be formed of a sheet of hydrophilic polypropylene spun bond material forming a fiber surface, with a hydrophobic polypropylene/polyethylene extruded seal layer on the other side. The spun bond acts as a wicking material 25. The polypropylene/polyethylene extruded seal layer forms the non-permeable side 24 of the sheet. In this case, the material is formed in two steps. First the spun bond fiber (wicking material 25) is formed. Second, a poly film is extruded on one side to form non-permable side 24.

The evaporation rate off of hydrophobic woven or spun bond material where water has been impregnated in between the fibers is higher then from a hydrophilic material where water has been absorbed into the material and between the fibers. This means that a much smaller temperature difference across the plate is required to achieve the same evaporation rate to take place, which therefore increases the heat transfer rate. Practically this means the indirect evaporative cooler of the present invention can realize the same temperature output with ⅓ higher air flow rates when using a hydrophilic polymer material. At the same time this hydrophobic material has the benefit of being able to wick water at a much faster rate as it does not absorb the water or hold onto the water; rather it allow it to quickly pass through the fibers.

Note that because the material is hydrophilic it will not naturally start the wicking process or absorb water, which may create the need for a wetting agent such as a surfactant to start the wicking. Air flow may also be used to help drive the water into the fibers. After the wicking is started it will continue to wick long after the surfactant has washed out, as long as the polymer wicking material is not allowed to run out of water to wick. To restart wicking after the wicking material has dried out, surfactant may be added to the evaporative fluid 22 for a brief period. The surfactant may be a detergent or hand soap or dish washing soap such as Dawn Ultra™.

Using this hydrophobic polymer material in a dew point evaporative cooler allows for thinner or less weight of material, lower product temperatures and higher air flow rates to be derived with the same surface area and air properties when compared to previous dew point evaporative coolers.

In step 304 plates 6 are cut from the material. In step 306, perforations are formed. In step 308, seal are added at the inlet and outlet ends of plates 6. In a preferred embodiment, an auto die cutter is used for steps 304-308. The die cutter uses a hot cutting surface, meaning that a narrow seal is formed on all of the edges and around the perforations as the plates are cut out. Seal strips 18 are also formed by the die cutter, by pressing the material between two plates, one of which is heated to about 320° F. The seal strips are about ¼″ wide.

In steps 310 and 312, channel guides are added to the plates 6B and 6A as showns in FIGS. 2B and 2C. The channel guides are formed by laying down hot melt glue with an automated machine for consistency to get the desired location, channel height and attachment between plates. In step 314, the plates are stacked, alternating between dry plates and wet plates as shown in FIG. 2D. In the embodiment of FIGS. 4A-D, in step 316 a trough is formed in the centers of the plates for example by fitting the plates into a shaped cassette

FIG. 4 is a perspective and schematic representation of another embodiment of the present invention, having slanted edge extensions. This embodiment is very similar to that of FIGS. 2A-2D, and much of the description is applicable here. Again seal strips 18 are formed at the inlet and outlet of the plates 6. Wet channel guides 7A are formed parallel to the seals, with the end guides overlapping the seals. Dry side channel guides 7B are formed perpendicular to the wet side channel guides.

Those skilled in the art of indirect evaporative cooling systems will recognize various changes and modifications which can be made to the exemplary embodiments shown and described above, which are still within the spirit and scope of the invention. 

1. A heat exchanger plate for use in an indirect evaporative cooling system, the plate comprising: a dry side having low permeability to an evaporative liquid and formed to allow a product fluid to flow over a heat transfer area of its surface; and a wet side designed to have its surface wet by an evaporative liquid, and formed to allow a working gas to flow over its surface to evaporate the evaporative liquid; wherein the wet side comprises a hydrophobic material formed to wick the evaporative fluid.
 2. The plate of claim 1, further comprising heat seal strips formed at an inlet edge and an outlet edge of the plate.
 3. The plate of claim 1 wherein the wet side comprises a spun bond material and the non-permable material comprises an extruded layer.
 4. The plate of claim 3 wherein the spun bond material is polypropylene and the non-permeable material is polypropylene/polyethylene.
 5. The plate of claim 1, further comprising channel guides to channel the working gas and the product fluid.
 6. The plate of claim 1 wherein the dry side is hydrophilic.
 7. An indirect evaporative cooler comprising: a plurality of generally parallel, spaced apart plates wherein each plate has a dry side having low permeability to an evaporative liquid and formed to allow a product fluid to flow over a heat transfer area of its surface; a wet side designed to have its surface wet by an evaporative liquid, and formed to allow a working gas to flow over its surface to evaporate the evaporative liquid; and; wherein the wet side comprises a hydrophobic material configured to wick the evaporative liquid; and wherein the plates alternate such that wet sides face wet sides and dry sides face dry sides.
 8. The indirect evaporative cooler of claim 7 wherein the plates are oriented generally horizontally and further comprise heat seal strips formed at an inlet edge and an outlet edge of the plates.
 9. The indirect evaporative cooler of claim 7, wherein the plates further form a trough containing the evaporative fluid between wet sides.
 10. The method of fabricating heat exchanger plates for use in an indirect evaporative cooling system comprising the steps of: (a) forming a sheet of material having a dry side of the sheet which has low permeability to an evaporative liquid and is configured to allow a product fluid to flow over a heat transfer area of its surface; and a wet side of the sheet which is hydrophobic and is configured to wick the evaporative fluid and to allow a working gas to flow over its surface to evaporate the evaporative liquid; and (b) cutting the sheets into plates wherein each plate has an inlet edge opposite an outlet edge;
 11. The method of claim 10, further comprising the steps of: (c) forming wet channels on wet sides of plates generally parallel to the inlet and outlet edges; and (d) forming dry channels on dry sides of plates.
 12. The method of claim 10 further comprising the step of forming heat seal strips at the inlet and outlet edges.
 13. The method of claim 12 wherein the step of forming wet channels comprises forming wet channel guides, and wherein a wet channel guide overlaps the heat seal strip at the inlet edge and a wet channel guide overlaps the heat seal strip at the outlet edge.
 14. The method of claim 10, further comprising the step of forming perforations in the sheets.
 15. The method of claim 14, wherein the step of forming dry channels comprises forming channel guides generally perpendicular to the inlet edge and the outlet edge, and where the step of forming perforations forms perforations in a line perpendicular to the inlet edge and outlet edge.
 16. The method of claim 10 wherein the step of forming further forms the dry side to be hydrophilic. 